1. Field of the Invention
The present invention relates to an islanding detection and protection method and, more particularly, to a method of performing perturbation to a power system to quickly detect islanding operation of the power system and trigger the protection mechanism for the power system.
2. Description of Related Art
Liberation of the power industry is the future trend of power development. Small distributed energy or renewable energy generating equipment such as cogeneration systems, solar energy generating systems and wind energy generating systems are usually incorporated into the public power system for parallel operation. When the public power system trips due to an abnormality, if the distributed energy generating equipment cannot detect that the public power system has tripped and still provides power, a stand-alone state will arise to form an islanding zone, which is called an islanding operation.
The key factor is that each distributed energy generating equipment usually belongs to a private owner but is not directly controlled by the power company. Therefore, when the public power system breaks off, if each distributed energy generating equipment cannot detect this situation and still provides power, the voltage and frequency will be unstable which leads to the damaging of electric equipment or even causes electric shock hazards to power maintenance men. Furthermore, when the public power system is restored, the distributed energy generating equipment or electric equipment may be damaged due to asynchronization.
Therefore, there are many documents concerning detection of an islanding operations, many of which are base on detection of the magnitude and frequency of voltage at the point of the common coupling (PCC) terminal of the power system. As shown in FIG. 1, once the voltage or frequency exceeds (beyond or below) the operating window, the distributed power system is disconnected from the public power system to avoid an islanding operation; or the voltage phase of the power system is detected momentarily to detect whether there is a voltage phase jump, representing the occurrence of an islanding operation.
The above methods of detecting voltage, frequency or phase jump are usually restricted by the form of load, and may be unable to detect the state of an islanding operation due to the load state. This is called a non-detection zone (NDZ).
Another method is to detect the total harmonic distortion (THD) of the PCC. When this THD exceeds a preset level, it represents the occurrence of an islanding operation. This method is based on the nonlinear hysteresis of the B-H curve of a transformer. When the power system operates normally, this nonlinear current is provided by the power system and does not affect the voltage harmonics. When the power system trips, this nonlinear current is provided by the distributed power system. At this time, high-order harmonics of voltage will be generated. Therefore, the THD can be used as a base to determine whether an islanding operation is occuring or not.
However, it is difficult to obtain the magnitude of the THD to determine whether an islanding operation has occurred or not. A method of using the power line carrier for direct communication to determine whether islanding operation occurs can also be used. However, it is necessary to simultaneously install communication equipment at both ends, hence causing additional expenses. The above methods are commonly called the passive islanding detection method.
Another important detection method is called the active anti-islanding detection method. For instance, in the detection technique developed by the Sandia National Lab, the frequency or magnitude of the output current of a power regulator is adjusted to perturb the power system, and the frequency or magnitude of the voltage of the power system is detected. When there is any change in the frequency or magnitude of the voltage of the power system, the frequency or magnitude of the output current of the power regulator is varied by means of positive feedback so as to accomplish the object of detecting an islanding operation.
If the power system operates normally, this small component won't affect the magnitude of the voltage. When the power system trips, the produced voltage change will enlarge the voltage drift through positive feedback. Therefore, whether an islanding operation occurs or not can be determined according to whether the voltage exceeds the operating window or not.
Using solar power regulators as an example, when a plurality of solar power regulators is parallel connected, the influences to the voltage drift may easily cancel out one another because a certain one has a smaller sunlight illumination while another has a larger sunlight illumination. Moreover, general solar power regulators operate near the load point with maximum power, but cannot provide a larger output current to perturb the voltage of the power system. Therefore, the phenomenon of an islanding operation is difficult to detect.
Another islanding detection method, called Sandia Frequency Shift (SFS), developed by the Sandia National Lab makes use of a frequency positive-feedback system. As shown in FIG. 2, the current frequency fn2 (E) is a curve equation that describes the relation between an error and an increment/decrement of the frequency.
When the power system operates normally, the change of the current frequency won't cause any drift in the voltage of the power system. If the power system trips, the voltage frequency will drift due to the change in the current frequency. Through positive feedback, the frequency of the output current will increase to exceed the normal operating window (too large or too small). An islanding operation can thus be detected.
With passive components (an inductor, a capacitor and a resistor) as the load, the lag angle is:arg tan(R−1+j ωc−j( ωL)−1)  (1)
From (1), if the operating frequency point is at the resonance point of the inductor and the capacitor, the error angle will be zero, and there is no drift in the frequency of the output current, hence not being able to detect an islanding operation. Furthermore, the detection time is too long to meet standards.
As shown in FIG. 3, U.S. Pat. No. 5,493,485 discloses a protection device for stopping operation of an inverter, which makes use of such factors as current and voltage phase drift, frequency variation, voltage variation and THD variation to determine the phase drift between the output current and voltage. Moreover, the equation describing the relation between frequency and phase difference can be among various kinds of nonlinear functions. Because the decision algorithm is complex, this system is hard to realize.
Additionally, there are some methods making use of the sliding mode of frequency and phase difference to adjust the phase difference between the injected current and the voltage when any drift in the voltage frequency is detected, thereby making the operation point reach another stable point along this sliding curve. This stable point lies outside the normal operating window. This sliding curve is shown in FIG. 4. Similarly, if the operating frequency point is at the resonance point of the inductor and the capacitor, the error angle will be zero, and there is no drift in the frequency of the output current, hence not being able to detect an islanding operation. Furthermore, the detection time is too long to meet standards.
Accordingly, the present invention aims to propose an islanding detection and protection method and system to solve the above problems in the prior art. In the present invention, the magnitude of error is converted to an error differential directional component (S value) and a counter (weighting factor Ws). The weighting factor Ws is used to adjust the variation of a negative-sequence current of a distributed power system to avoid the problem that the adjustment diminishes with decrease of the feedback error, thereby accurately and quickly detecting the occurrence of an islanding operation.